Processes and apparatus for breaking off the marginal strip of a glass blank laterally outward of a score defining the contour of a glass pane are known in the prior art. These processes, as the present process and the apparatus to be described, have particular utility in the fabrication of glass panes suitable for use in automobiles.
German Offenlegungsschrift Nos. 27 24 238 and 28 26 694 as well as U.S. Pat. No. 3,913,812 describe processes which generally relate to the application of tensile stress within the area of the score defining the contour of the glass pane for purposes of breaking off the strip laterally outward of the score. Typically, the glass blank whose marginal edge portions are broken away whereby the glass pane remains is supported by a rigid tool in the area below the score and pressure is applied by pressure tools lowered from above and acting on both sides of the score.
One problem and disadvantage with regard to apparatus of this type is that the supporting tool required in use must be adapted to the contour of the glass pane from which the lateral edge portions are broken. What this means is that for every change in contour of a glass pane it is required that the apparatus be converted and aligned to accommodate that change. Thus, individual supporting tools for use in the fabrication of a glass pane of particular contour must be located to the apparatus for accomplishing the end result with consequent expenditure, which may be considerable, in the required change as well as loss through idle time during the changeover period.
The prior art also includes forms of apparatus used in the fabrication of a glass pane which do not require supporting tools having a contour which generally duplicates the contour of the glass pane to be fabricated. A typical apparatus of this type is disclosed in U.S. Pat. No. 3,532,259. Particularly, the glass blank is supported on an elastically resilient support which may be in the form of a belt comprising the belt of a conveyor. The belt rests on a level metal plate and tensile stresses are developed by means of a plurality of pressure tools which are lowered toward the glass blank from an elevated position. The pressure tools which contact the surface of the glass blank may have a diameter of approximately 30 mm. The pressure tools are disposed in a manner so that they are capable of acting upon the glass blank along the score when lowered from the elevated position. According to this process as disclosed in the aforementioned patent, the surface of the glass blank supported by the elastically resilient support undergoes tensile stress through action of the pressure tools and through this action the score defining the contour of the glass pane undergoes compressive stress. The bending moment, accordingly, is opposite, it may be considered to have an opposite sign from the bending moment which the glass blank undergoes in the first described process and apparatus. It has been found that a negative bending moment, particularly if the thickness of the glass blank is in excess of 3 mm, is not satisfactory in the fabrication of a glass pane. To this end, it has been found that it is difficult to cause the marginal edge portions laterally outward of the score to break away from the glass pane by a neat, true break.